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Regularly interstratified chlorite/smectite (corrensite) is rarely found in hydrocarbon-bearing reservoir rocks as a dominant constituent of the clay mineral suite. However, several sandstone core samples, when subject to X-ray diffraction (XRD) analysis, were found to contain 46-100% of this mixed-layered clay mineral. The samples containing 100% corrensite permitted characterization of crystal morphology and mode of occurrence by use of scanning electron microscopy (SEM).
Corrensite was identified by XRD analysis because its characteristic basal reflections as the mixed-layered clay mineral responded to glycolation and heating treatment. This mineral consists of a 14A chlorite in a 1:1 relation with a 15A expandable smectite layer, yielding a total thickness of 29A. Glycolation expands the swelling smectite layer to 32.7A (001), 16.0A (002), 7.97A (003), and 5.91A (004).
The identification of corrensite as the dominant clay mineral of this reservoir rock is significant in that it has permitted: (1) characterization of the crystal morphology of this mixed-layered clay mineral by SEM; (2) the definition of a part of the pore-lining clays in a sandstone reservoir rock as water-sensitive due to the expandable smectite layers; (3) identification of chlorite within this mixed-layered clay so that proper completion fluids could be added to chelate the iron released if hydrocloric acid was used to stimulate the formation; and (4) differentiation of this type of corrensite from chlorite-swelling chlorite and chlorite-vermiculite, and other mixed-layered minerals to insure proper reservoir exploitation.
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